Code Modernization: Fix "passing null to non-nullable" deprecation from next_posts().

The `esc_url()` function expects to a string for `$url` parameter. There is no input validation within that function. The function contains a `ltrim()` which also expects a string. Passing `null` to this parameter results in `Deprecated: ltrim(): Passing null to parameter #1 ($string) of type string is deprecated` notice on PHP 8.1+.

Tracing the stack back, a `null` is being passed to it within `next_posts()` when `get_next_posts_page_link()` returns `null` (it can return a string or `null`).

On PHP 7.0 to PHP 8.x, an empty string is returned from `esc_url()` when `null` is passed to it. The change in this changeset avoids the deprecation notice by not invoking `esc_url()` when `get_next_posts_page_link()` returns `null` and instead sets the `$output` to an empty string, thus maintain the same behavior as before (minus the deprecation notice).

Adds a test to validate an empty string is returned and the absence of the deprecation (when running on PHP 8.1+).

Follow-up to [11383], [9632].

Props codersantosh, nihar007, hellofromTonya, mukesh27, oglekler, rajinsharwar.
Fixes #59154.

git-svn-id: https://develop.svn.wordpress.org/trunk@56740 602fd350-edb4-49c9-b593-d223f7449a82

Author: hellofromtonya

src/wp-includes/link-template.php

@@ -2511,7 +2511,8 @@ function get_next_posts_page_link( $max_page = 0 ) {
  * @return string|void The link URL for next posts page if `$display = false`.
  */
 function next_posts( $max_page = 0, $display = true ) {
-	$output = esc_url( get_next_posts_page_link( $max_page ) );
+	$link   = get_next_posts_page_link( $max_page );
+	$output = $link ? esc_url( $link ) : '';
 
 	if ( $display ) {
 		echo $output;

tests/phpunit/tests/link/nextPosts.php

@@ -0,0 +1,41 @@
+<?php
+
+/**
+ * Tests the `next_posts()` function.
+ *
+ * @since 6.4.0
+ *
+ * @group link
+ *
+ * @covers ::next_posts
+ */
+class Tests_Link_NextPosts extends WP_UnitTestCase {
+
+	/**
+	 * Creates posts before any tests run.
+	 *
+	 * @param WP_UnitTest_Factory $factory
+	 */
+	public static function wpSetUpBeforeClass( WP_UnitTest_Factory $factory ) {
+		global $wp_query, $paged;
+
+		$factory->post->create_many( 3 );
+		$paged    = 2;
+		$wp_query = new WP_Query(
+			array(
+				'post_type'      => 'post',
+				'posts_per_page' => 1,
+				'paged'          => $paged,
+			)
+		);
+	}
+
+	/**
+	 * The absence of a deprecation notice on PHP 8.1+ also shows that the issue is resolved.
+	 *
+	 * @ticket 59154
+	 */
+	public function test_should_return_empty_string_when_no_next_posts_page_link() {
+		$this->assertSame( '', next_posts( 1, false ) );
+	}
+}